In various relief printing areas including newspaper printing, flexographic printing material has been widely used. Such material is usually constructed as a laminate comprising, in successive order, a support, a photosensitive layer consisting essentially of a developer soluble, high molecular weight binder, a polymerizable unsaturated monomer or monomers and a photoinitiator, slip layer of a high molecular weight substance for the purpose of improving a close contact of the photosensitive resin layer to an original, a releasing layer and a protective film or sheet.
On the use of such material, the protective film or sheet and the releasing layer are peeled out to expose the slip layer of the laminate. An original is then placed on said slip layer and exposed to an actinic radiation for a desired time to effect photo-curing (or setting) of the exposed resin layer.
The exposed material is then treated with a developer to remove the unexposed, uncured photosensitive resin and the slip layer to obtain a printing plate having a relief exactly copying the image in the original.
Therefore, from a standview of light penetration, it is preferred that the photosensitive layer is as clear as possible, and, in fact, many of the commercialized printing materials do bear a transparent photosensitive resin layer. Light goes through the resin layer. However, it is well know that when a light beam passes through a medium with a certain thickness, there often occurs irregular reflection or refraction of the light depending on the nature of said medium. In the preparation of a relief printing plate using a photosensitive resin base printing material, such irregular reflection or refraction of an incident light is believed to be important for the formation of shoulder of the relief image.
However, excessive amounts of irregular reflection or refraction are undesirable because it would cause photocuring, in certain degree, at the unexposed area of the photosensitive layer (the so-called fogging phenomenon), and hence, a deficient dissolving of the photosensitive layer at the unexposed area in a development stage.
More concretely speaking, at the time when a half-tone negative with various sizes of dots is closely contacted as an original with a transparent photosensitive resin layer and exposed to a light, there are indeed no particular problems at the portion of said photosensitive resin layer where it is contacted with negative dots having comparatively smaller dot-area (the so-called high-light area of letterpress), but at the portion where the photosensitive resin layer is contacted with negative dots having comparatively larger dot-area (the so-called shadow area of letterpress), there arises the so-called fogging phenomenon, thereby resulting in the loss of shoulder under the influence of irregular reflection of incident light and reflected light from a support. As a consequence, a shallow depth image is transformed at the shadow area of the printing plate and a solid printing is results.
Attempts have therefore been made to provide an antihalation layer containing a pigment or dyestuff which will absorb visible or ultraviolet rays used in an exposure stage, between a photosensitive resin layer and a support, thereby absorbing the reflected light from the support and reducing the undesired influence of the reflected light toward shadow area. However, in that case, the desired amounts of reflected light for high-light area are likewise absorbed and there often arises the case wherein high-light area, fine letter or fine dot area are exposed deficiently. To compensate for this, a longer time exposure is required.
Moreover, it is essential that a non-diffusing coloring matter should be selected. This is because, if the coloring matter is of diffusible nature there is a tendency that the coloring matter used would migrate into a lower portion of the photosensitive layer through diffusion, whereby the reflected light desired for photo-curing of resin at that portion is objectionably absorbed. Thus, there occurs a side-edging of the image at high-light area, fine letter or fine dot area and, in an extremely case, undesired falling off the relief image.
In other attempts, such coloring matters are directly added to and uniformly kneaded with a photosensitive resin composition to be used for the preparation of photosensitive layer. However, in this case, an incident light is inevitably absorbed by the photosensitive layer itself in regular succession as it goes through said layer.
Therefore, there is a severe side-edging of the relief image at the high-light area and the like and falling off of the desired image. Even if a long time of exposure is adopted, there are various troubles such as fogging and the like at the shadow area, in return.
Thus, a photosensitive resin base printing material capable of resulting high quality images both in shadow area, where it is easily affected by irregular reflection of incident light and reflected light from a support at a comparatively upper portion of a photosensitive layer, and in high-light area, where it is easily affected by transmitted light and reflected light from a support at a comparatively lower portion of the photosensitive layer, has never been found.
It is, therefore, an object of the present invention to provide a printing material capable of resulting high quality images both in high-light and shadow areas, through conventional processes of photo-exposure and development. An additional object of the invention is to provide a printing material having an improved resolution and being useful in a relief printing area.